Semiconductor materials and elements that emit light of a shorter wavelength have come to be required of semiconductor light emitting devices in recent years. In particular, elements with a large bandgap, that is, that emit ultraviolet light with a wavelength of roughly 400 nm or less, are expected to find use in a wide range of applications because they can be used as the light source of photocatalysts and can provide a sterilizing function.
GaN, AlN, ZnO, and diamond are among the known semiconductor materials that emit ultraviolet light. The bandgaps of these materials and the corresponding emission wavelengths are 3.39 eV and 366 nm for GaN, 6.2 eV and 200 nm for AlN, 3.35 eV and 370 nm for ZnO, and 5.47 eV and 227 nm for diamond. With an Al—Ga—N ternary semiconductor, the values can be varied between 3.3 and 6.2 eV and between 200 and 366 nm. The last few years have seen considerable applied research into light emitting diodes and laser diodes made from these semiconductors, as well as applied research into light receiving elements (photodiodes).
“Ultraviolet light” generally refers to electromagnetic waves with a wavelength of about 100 to 400 nm, and is classified by wavelength into UV-A (325 to 400 nm), UV-B (280 to 325 nm), and UV-C (100 to 280 nm). The portion of UV-C from 100 to 200 nm is called vacuum ultraviolet light. Radiation of 254 nm is known to have a powerful sterilizing action because it directly destroys the DNA of viruses, bacteria, and so forth, and is therefore used in UV lamps. Radiation of 180 to 254 nm is useful in water treatment, such as the cleaning of sewage. In addition, radiation of 333 to 364 nm is widely used for photolithography, and that of 200 to 400 nm for the curing of UV-setting resins. At present, these ultraviolet rays are mainly generated by mercury vapor lamps. The use of semiconductor light emitting diodes in place of mercury vapor lamps has recently been studied as a way to avoid using mercury, which is harmful to the environment, and some products have already seen practical use.
Meanwhile, an ultraviolet light source is also needed for photocatalysts whose main component is TiO2 or the like. Photocatalysts are mainly composed of TiO2 microparticles, and when irradiated with ultraviolet light, they generate oxygen radicals that react with and decompose the molecules that make up stains and organic matter. Photocatalysts have been used for sewage cleaning and air purification equipment, toxic gas decomposition apparatus, and so on. To obtain a photocatalytic action, the catalyst must be irradiated with ultraviolet light having an energy of at least 3.2 eV (equivalent to a wavelength of 388 nm or less), which is the bandgap of TiO2 (anatase), and here again black lights and other such mercury vapor lamps have been used. And, semiconductor light emitting diodes have also been studied, with some being put to practical application. Photocatalysts that function with visible light have also been invented. With these, part of the TiO2 in the material is doped with nitrogen, so that the material is excited by visible light of 400 to 500 nm and thereby exhibits a photocatalytic action. The effect, however, is weaker than that of a UV-excited type of photocatalyst.
To efficiently sterilize viruses and bacteria, as well as organic matter, these must first be trapped, and the trapped material must be collected together and then irradiated with ultraviolet light. The reason is that ultraviolet light attenuates in the air and in liquids. Particularly in the treatment of sewage and other liquids in which there is a large amount of suspended matter, the reach is extremely short, so UV irradiation is performed after the suspended matter has first been precipitated or filtered off with a filtration membrane. In gases, either a nitrogen atmosphere that has a low UV attenuation factor is used, or a mercury vapor lamp with high output is used to increase the reach. These methods are drive up the cost, however, which poses a serious obstacle to their practical application.
In recent years there has been an increasing need for filters to be ceramic filters that offer higher heat resistance, strength, and permeability. Such ceramic filters have been used, for example, in the fields of food and pharmaceuticals. Organic membranes used to be used in these fields, but ceramics offer heat resistance, pressure resistance, chemical resistance, and high separability not available with organic membranes, and are gradually supplanting organic membranes. Furthermore, porous membranes have been used, for example, as catalyst carriers or bioreactors for microbe cultivation carriers, and so on.
One of the various types of ceramic available, silicon nitride is a structural ceramic material that has high strength, toughness, thermal shock resistance, and chemical resistance, and is therefore extremely promising as a filter material. A filter consisting of porous Si3N4 has been invented, in which Si3N4 particles having a columnar structure are bonded together so as to form a three-dimensionally intertwined structure by means of a binder phase containing at least one of compound of rare earth element (which refers to scandium, yttrium, and lanthanide elements).
For example, in Japanese Patent No. 2,683,452, it is indicated that porous Si3N4 in which columnar Si3N4 crystal particles are randomly oriented via an oxide-based binder phase has high strength characteristics and exhibits high permeability when used as a filter. This porous Si3N4 is manufactured by the following process. An oxide of a rare earth element (used as a sintering auxiliary) and an Si3N4 powder are mixed in specific proportions and then molded and fired in an inert gas. “Rare earth element” refers to scandium, yttrium, and the elements with atomic numbers 57 to 71. When Y2O3 is used as an auxiliary, for instance, it is understood that columnar Si3N4 particles are grown and a porous structure is produced when the Y2O3 and the SiO2 present at the surface of the raw material Si3N4 form a liquid phase at the firing temperature, in which part of the Si3N4 dissolves and is reprecipitated.
The above-mentioned Si3N4 filters are the same as ordinary filters in that their only function is to filter according to the pore size of the porous material. Specifically, particles of organic components, bacteria, viruses, and so forth that are smaller than the pores cannot be trapped by filtration. The only method available for trapping these and obtaining a clarified permeated liquid has been to make the pore size of the porous material smaller than these particles, bacteria, and viruses. When the pore size is decreased, however, the problem is that there is more pressure loss in the filtration process, and this greatly compromises the permeation performance. Another drawback is that if part of the porous material breaks apart and the pores become larger, bacteria and the like can become admixed into the permeated liquid.
In addition, the following problems were encountered with ultraviolet light emitting porous materials produced by the prior art discussed above.
Difficult Pore Control
Methods employed up to now for rendering porous a semiconductor having a wide bandgap were not easy because in every case controlling the pore size of the porous material entailed so many steps.
Low Strength
A ceramic that has been rendered porous by prior art has weak bonds between the particles, so its strength is inadequate.
Low Permeability
Permeability is low when a porous material comprising semi-bonded spherical particles is used as a filter.
Low Thermal Thermal Shock Resistance
Because strength is low, thermal shock resistance is also low.